Protein kinases participate in the signalling events which control the activation, growth and differentiation of cells in response to extracellular mediators and to changes in the environment. In general, these kinases fall into several groups; those which preferentially phosphorylate serine and/or threonine residues and those which preferentially phosphorylate tyrosine residues [S. K. Hanks and T. Hunter, FASEB. J., 1995, 9, pages 576-596]. The serine/threonine kinases include for example, protein kinase C isoforms [A. C. Newton, J. Biol. Chem., 1995, 270, pages 28495-28498] and a group of cyclin-dependent kinases such as cdc2 [J. Pines, Trends in Biochemical Sciences, 1995, 18, pages 195-197]. The tyrosine kinases include membrane-spanning growth factor receptors such as the epidermal growth factor receptor [S. Iwashita and M. Kobayashi, Cellular Signalling, 1992, 4, pages 123-132], and cytosolic non-receptor kinases such as p56tck, p59fYn, ZAP-70 and csk kinases [C. Chan et. al., Ann. Rev. Immunol., 1994,12, pages 555-592].
Inappropriately high protein kinase activity has been implicated in many diseases resulting from abnormal cellular function. This might arise either directly or indirectly, for example by failure of the proper control mechanisms for the kinase, related for example to mutation, over-expression or inappropriate activation of the enzyme; or by over- or underproduction of cytokines or growth factors also participating in the transduction of signals upstream or downstream of the kinase. In all of these instances, selective inhibition of the action of the kinase might be expected to have a beneficial effect.
Syk is a 72-kDa cytoplasmic protein tyrosine kinase that is expressed in a variety of hematopoietic cells and is an essential element in several cascades that couple antigen receptors to cellular responses. Thus, Syk plays a pivotal role in signalling of the high affinity IgE receptor, FcεR1, in mast cells and in receptor antigen signalling in T and B lymphocytes. The signal transduction pathways present in mast, T and B cells have common features. The ligand binding domain of the receptor lacks intrinsic tyrosine kinase activity. However, they interact with transducing subunits that contain immunoreceptor tyrosine based activation motifs (ITAMs) [M. Reth, Nature, 1989, 338, pages 383-384]. These motifs are present in both the β and γ subunits of the FcεR1, in the ξ-subunit the of T cell receptor (TCR) and in the IgGα and IgG β subunits of the B cell receptor (BCR). [N. S. van Oers and A. Weiss, Seminars in Immunology, 1995, 7, pages 227-236] Upon binding of antigen and multimerization, the ITAM residues are phosphorylated by protein tyrosine kinases of the Src family. Syk belongs to a unique class of tyrosine kinases that have two tandem Src homology 2 (SH2) domains and a C terminal catalytic domain. These SH2 domains bind with high affinity to ITAMs and this SH2-mediated association of Syk with an activated receptor stimulates Syk kinase activity and localises Syk to the plasma membrane.
In Syk deficient mice, mast cell degranulation is inhibited, suggesting that this is an important target for the development of mast cell stabilising agents [P. S. Costello, Oncogene, 1996, 13, pages 2595-2605]. Similar studies have demonstrated a critical role for Syk in BCR and TCR signalling [A. M. Cheng, Nature, 1995, 378, pages 303-306, (1995) and D. H. Chu et al., Immunological Reviews, 1998, 165, pages 167-180]. Syk also appears to be involved in eosinophil survival in response to IL-5 and GM-CSF [S. Yousefi et al., J. Exp. Med., 1996, 183, pages 1407-1414]. Despite the key role of Syk in mast cell, BCR and T cell signalling, little is known about the mechanism by which Syk transmits downstream effectors. Two adaptor proteins, BLNK (B cell Linker protein, SLP-65) and SLP-76 have been shown to be substrates of Syk in B cells and mast cells respectively and have been postulated to interface Syk with downstream effectors [M. Ishiai et al., Immunity, 1999, 10, pages 117-125 and L. R. Hendricks-Taylor et al., J. Biol. Chem, 1997, 272, pages 1363-1367]. In addition Syk appears to play an important role in the CD40 signalling pathway, which plays an important role in B cell proliferation [M. Faris et al., J. Exp. Med., 1994, 179, pages 1923-1931].
Syk is further involved in the activation of platelets stimulated via the low-affinity IgG receptor (Fc gamma-RIIA) or stimulated by collagen [F. Yanaga et al., Biochem. J., 1995, 311, (Pt. 2) pages 471-478].
Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase involved in integrin-mediated signal transduction pathways. FAK colocalizes with integrins in focal contact sites and FAK activation and its tyrosine phosphorylation have been shown in many cell types to be dependent on integrins binding to their extracellular ligands. Results from several studies support the hypothesis that FAK inhibitors could be useful in cancer treatment. For example, FAK-deficient cells migrate poorly in response to chemotactic signals and overexpression of C-terminal domain of FAK blocks cell spreading as well as chemotactic migration (Sieg et al, J. Cell Science, 1999, 112, 2677-2691; Richardson A. and Parsons T., Cell, 1997, 97, 221-231); in addition, tumor cells treated with FAK antisense oligonucleotides lost their attachment and underwent apoptosis (Xu et al, Cell Growth Differ. 1996, 4, 413-418). FAK has been reported to be overexpressed in prostate, breast, thyroid, colon and lung cancers. The level of expression of FAK is directly correlated with tumors demonstrating the most aggressive phenotype.
Angiogenesis or the formation of new blood vessels by sprouting from the preexisting vasculature is of central importance for embryonic development and organogenesis. Abnormal enhanced neovascularization is observed in rheumatoid arthritis, diabetic retinopathy and during tumor development (Folkman, Nat. Med., 1995, 1, 27-31.). Angiogenesis is a complex multistage process which includes activation, migration, proliferation and survival of endothelial cells. Extensive studies in the field of tumor angiogenesis in the past two decades have identified a number of therapeutic targets including kinases, proteases and integrins resulting in the discovery of many new anti-angiogenic agents, including KDR inhibitors some of which are currently under clinical evaluation (Jekunen, et al Cancer Treatment Rev. 1997, 23, 263-286.). Angiogenesis inhibitors may be used in frontline, adjuvant and even preventive settings for the emergence or regrowth of malignancies.
Several proteins involved in chromosome segregation and spindle assembly have been identified in yeast and drosophila. Disruption of these proteins results in chromosome missegregation and monopolar or disrupted spindles. Among these kinases are the Ip11 and aurora kinases from S. cerevisiae and drosophila respectively, which are required for centrosome separation and chromosome segregation. One human homologue of yeast Ip11 was recently cloned and characterized by different laboratories. This kinase termed Aurora2, STK15 or BTAK belongs to the serine/threonine kinase family. Bischoff et al showed that Aurora2 is oncogenic and is amplified in human colorectal cancers (EMBO J, 1998, 17, 3052-3065). It has also been exemplified in cancers involving epithelial tumors such as breast cancer.